Formation of Ba3Nb0.75Mn2.25O9-6H during thermochemical reduction of Ba4NbMn3O12-12R

Nearly complete conversion to a Ba3Nb0.75Mn2.25O9-6H structure was observed following thermochemical reduction of the parent Ba4NbMn3O12-12R material. The Ba3Nb0.75Mn2.25O9-6H structure represents a hexagonal perovskite that displays substitution of Mn onto Nb sites in order to satisfy the 3:1 Mn:Nb ratio within the 6H structural motif.

The resurgence of interest in hydrogen-related technologies has stimulated new studies aimed at advancing lesser-developed water-splitting processes, such as solar thermochemical hydrogen production (STCH). Progress in STCH has been largely hindered by a lack of new materials able to efficiently split water at a rate comparable to ceria under identical experimental conditions. BaCe 0.25 Mn 0.75 O 3 (BCM) recently demonstrated enhanced hydrogen production over ceria and has the potential to further our understanding of two-step thermochemical cycles. A significant feature of the 12R hexagonal perovskite structure of BCM is the tendency to, in part, form a 6H polytype at high temperatures and reducing environments (i.e., during the first step of the thermochemical cycle), which may serve to mitigate degradation of the complex oxide. An analogous compound, namely BaNb 0.25 Mn 0.75 O 3 (BNM) with a 12R structure was synthesized and displays nearly complete conversion to the 6H structure under identical reaction conditions as BCM. The structure of the BNM-6H polytype was determined from Rietveld refinement of synchrotron powder X-ray diffraction data and is presented within the context of the previously established BCM-6H structure.

Chemical context
Hydrogen production has received considerable attention because of the recent technological advances and initiatives directed towards increasing the yield and reducing the cost of hydrogen production over the next decade. Although electrolysis-based methods of hydrogen production present nearterm opportunities for widespread commercialization, incipient methods such as solar thermochemical hydrogen production (STCH) offer higher theoretical efficiencies with the possibility of integration with solar or waste-heat renewable energy sources. Ceria is currently the benchmark material for a direct 'two-step' thermochemical cycle. More recently, BaCe 0.25 Mn 0.75 O 3 (BCM) has demonstrated water-splitting properties commensurate with ceria at lower reaction temperatures (Barcellos et al., 2018). Known BCM structural phases (BCM-12R, -10H, and -6H polytypes) are related by variations in the [MnO 6 ] oligomer size (Fuentes et al., 2004;Macías et al., 2013;Strange et al., 2022). Previous work revealed the presence of a BCM-12R to -6H ([MnO 6 ] trimer to dimer) polytypic structural transformation during thermochemical reduction at 1350 C, potentially inhibiting decomposition into simpler refractory oxides at or near watersplitting conditions (Strange et al., 2022). Compositional analogues to BCM, such as BaNb 0.25 Mn 0.75 O 3 (BNM) are currently under investigation to determine their water-splitting efficacy and structural behaviors over ranges of environmental conditions. The BNM-12R structure was first reported by Nguyen et al. (2019), where the magnetic behavior of the Mn ions was the primary focus. In the present communication, an alternative pathway was utilized to synthesize the BNM-12R compound with a 92.2 wt% purity as determined by Rietveld refinement of synchrotron powder X-ray diffraction (see Fig. 1a). The high-purity BNM-12R material was subsequently exposed to low oxygen partial pressure conditions at 1350 C, where the powder exhibited nearly complete conversion to the previously unreported 6H-polytype of BNM (Fig. 1b), in contrast with BCM-6H, which displayed only partial conversion under identical experimental conditions.

Structural commentary
The BNM-12R structure (Fig. 2a) belongs to a family of hexagonal perovskites (Tilley, 2016) with alternating layers of face-sharing [MnO 6 ] trimers separated by ordered cornersharing [NbO 6 ] units and is associated with the Ba 2 NiTeO 6 structure type (Kö hl et al., 1972). BaO 3 layers are formed with a cubic-cubic-hexagonal-hexagonal, (cchh) 3 , stacking. In contrast to the fully stoichiometric BCM-12R structure reported by Fuentes et al. (2004), the analogous fully stoichiometric BNM-12R structure (with a 3:1 ratio of Mn:Nb) requires an average charge of at least +3.667 distributed across the two Mn sites since Nb remains entirely in oxidation state +5 during redox cycles. An average Mn oxidation state of 3.667+ in BNM-12R requires an Mn 4+ :Mn 3+ cation ratio of 2:1 on the B site. This finding suggests that the BNM-12R structure exists with at minimum 33% of the Mn cations initially in the 3+ oxidation state. Minor structural impurities consisted primarily of cubic BaNb 0.5 Mn 0.5 O 3 and (Ba 3 MnNb 2 O 9 ) 0.333 compounds, which contain Mn exclusively in the +3 and +2 oxidation states, respectively. Residual diffraction peaks were indexed to a 10H analogue of BNM (a = 5.725, c = 23.537 Å ), but the observed weight fraction of the species was too low to reliably refine additional parameters of the crystal structure. The three Mn-O bond lengths in BNM-12R are 1.876 (3) Å (6Â), 1.935 (3) Å (3Â), and 1.968 (3) Å (3Â), where the smallest distance corresponds to internal Mn1-O1 bonds within the trimeric unit. For comparison, Mn-O bond length in BaMnO 3 -2H (Cussen & Battle, 2000), which exhibits Mn 4+ cations within exclusively face-sharing [MnO 6 ] octahedra, is 1.904 Å . The Mn-O distances in BNM-12R are also systematically larger than the analogous bond lengths in BCM-12R (1.855, 1.928, and 1.955 Å ), which entirely displays Mn 4+ cations, thereby supporting inherent partial reduction in the as-synthesized BNM-12R compound.
The BNM-6H structure (Fig. 2b) is also a hexagonal perovskite, exhibiting [MnO 6 ] dimers, in contrast to the trimers found in BNM-12R, and is of the BaFeO 3-structure type (Grey et al., 1998). The dimers in BNM-6H are separated by [NbO 6 ] corner-sharing octahedra. BaO 3 units are found with (cch) 2 stacking. In order to conserve the 3:1 Mn:Nb stoichiometry, partial substitution of Mn onto the Nb sites is necessary. Since the fully stoichiometric BNM-12R structure contains at least 33% Mn 3+ cations, the formation of BNM-6H must be accompanied in part by the additional oxygen vacancies and further reduction from Mn 4+ ! Mn 3+ . The relative concentrations of structural impurities initially observed in the BNM-12R material were decreased during thermochemical reduction, accounting for only $2.4 wt% of the 'reduced' material. It is noteworthy that the BNM-10H polytype was no longer detectable following thermochemical reduction. The Mn-O bond lengths within BNM-6H are 1.930 (3) Å (3Â), 1.960 (3) Å (3Â), and 2.051 (3) Å (3Â) and are elongated relative to the BNR-12R [MnO 6 ] octahedra, establishing further reduction of Mn 4+ to Mn 3+ . The largest of these values is associated with the (Nb,Mn)-O distance and is dually impacted by Mn and Nb shared occupation and corner sharing [(Nb,Mn)O 6 ] octahedra. The (Nb,Mn)-O bond length in BNM-6H [2.051 (3) Å ] increases only slightly by $2.5% relative to the Nb-O distance without occupational disorder [2.001 (3) Å ] within BNM-12R during the 12R-to 6H-polytype transformation.
The BNM-12R structure has been investigated for competing ferromagnetic and antiferromagnetic interactions between neighboring trimers of [MnO 6 ] octahedra (Nguyen et al., 2019). The BNM-6H structure discussed herein has substantial substitution of Mn onto Nb sites, potentially leading to a distribution of Mn dimers, pentamers, and decreasing occurrence of higher n chains of 3n + 2 [MnO 6 ] octahedra. The influences of these different length chains of [MnO 6 ] octahedra on the magnetic properties are a subject of potential interest for these materials.

Database survey
A query was made to the Inorganic Crystal Structural Database (ICSD, version 4.8.0;Zagorac et al., 2019) to search for related crystal structures within 1% of the reported BNM-6H lattice parameters. Limiting the results to structures with P6 3 / mmc space-group symmetry, a series of BaMO 3 (M = transition metal) compounds were identified. For many of these structures, the B-sites of the ABO 3 compounds are shared between two metals, often with a 2:1 ratio, which satisfies full occupation of atomic sites for A and B site cations (i.e., site mixing is not necessary to achieve the compound stoichiometry). The cation with a larger ionic radius is typically the less abundant species in the 2:1 ratio. The reported BNM-6H structure is one of few structures within this series to display a 3:1 mixing ratio on the B-site and is the only structure where mixing is achieved exclusively by the smaller cation (e.g., Mn) substituting onto the larger cation (e.g., Nb) B-site position. In the related BaFe 0.25 Ti 0.75 O 3 structure, mixing of B-site cations is present on both the 2a and 4f Wyckoff sites. Furthermore, BNM-6H is the only reported structure to display a combination of cations with oxidation states of 5+ (Nb) and 4+/3+ (Mn). The metal-oxygen bond lengths and angles among these BaMO 3 structures are nominally equivalent (with exception to non-stoichiometric states). The shorter cation bonds with oxygen are slightly less than 2 Å , whereas the longer cation bonds with oxygen are slightly greater than 2 Å . The O-Ba-O and O-M-O bond angles reside near their respective ideal values. For the synthesis of BaNb 0.25 Mn 0.75 O 3 -12R (5 g scale reaction), barium nitrate (5.2319 g), niobium (V) oxide (0.6651 g), manganese (II) nitrate tetrahydrate (3.7686 g), and anhydrous citric acid (6.3101 g) at a molar ratio of 1:0.25:0.75:1.5 were suspended in $25 ml of deionized water. Most of the water was evaporated on a hot plate while stirring until a viscous liquid was obtained. This was dried at 110 C in air overnight, ground into a powder, then self-combusted on a hot plate. The resulting powder was ground, calcined at 800 C (5 C min À1 ) in air for 12 h, then sintered at 1300 C (10 C min À1 ) for another 12 h with intermediate grinding. The calcination temperature was determined from the previously reported solid-state synthesis method (Nguyen et al., 2019).

Synthesis and crystallization
Thermogravimetric Analysis The thermogravimetric analysis experiment was performed using a Netzsch STA 449 F1 Jupiter thermal analyzer under gas flow rates of 100 ml min À1 . Baseline correction was performed on an empty crucible with no sample. Argon gas (Matheson, UHP grade) and air (Matheson, ultra-zero grade) were used as received. Before reduction, the as-synthesized Ba 4 NbMn 3 O 12 -12R powder was initially redox-cycled three times. For this, the sample was heated to 1350 C (10 C min À1 ) under Ar, held isothermally for 30 minutes, cooled to 400 C (25 C min À1 ), then held isothermally for 30 minutes (see Fig. 3). With the gas changed to a mixture of air (80%) and Ar (20%), the sample was heated to 1100 C (10 C min À1 ), held isothermally for 30 minutes, then cooled to 200 C (25 C min À1 ). For repeated cycles, the sample was re-weighed between runs.
The redox-cycled Ba 4 NbMn 3 O 12 -12R was reduced with an oxidation step included in the TGA experiment prior to the reduction to determine the mass reference point at which the sample was fully oxidized. For this, the sample was heated under air to 1100 C (20 C min À1 ), held isothermally for 30 minutes, cooled to 200 C (20 C min À1 ), then held isothermally for 30 minutes. For the reduction, the sample was heated under Ar to 1350 C (20 C min À1 ), held isothermally for 2 h, cooled to 50 C (50 C min À1 ), then held isothermally for 30 minutes. The atmosphere was then changed to air to ensure no mass gain, indicating oxidation, was observed. The thermogram shown is baseline-corrected with the initial oxidation step (for mass reference point) omitted.

Refinement
Synchrotron powder X-ray diffraction (SPXRD) measurements were performed at the Stanford Synchrotron Radiation Lightsource (SSRL) beam line 2-1 on two BNM materials. The as-synthesized BNM powder, referred to as 'pristine BNM', targeted a pure BNM-12R phase. The second sample, referred to as 'reduced BNM', was produced by thermochemically reducing the pristine BNM powder as detailed in the thermogravimetric analysis sub-section above. The two BNM samples were prepared in 0.5 mm (0.01 mm wall thickness) glass capillaries. XRD data was acquired using a Pilatus 100K hybrid photon counting detector with portrait orientation. Two-dimensional detector images were normalized to incident beam intensity, stitched, and integrated into one-dimensional diffraction data using a Python script developed at SSRL, specifically for beam line 2-1. Crystal data, data collection and structure refinement details are summarized in Table 1.